The healthy heart produces regular, synchronized contractions. Rhythmic contractions of the heart are normally controlled by the sinoatrial (SA) node, specialized cells located in the upper right atrium. The SA node is the normal pacemaker of the heart, typically initiating 60-100 heart beats per minute. When the SA node is pacing the heart normally, the heart is said to be in normal sinus rhythm (NSR).
If heart contractions are uncoordinated or irregular, the heart is denoted to be arrhythmic. Cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event. Cardiac arrhythmias have a number of etiological sources including tissue damage due to myocardial infarction, infection, or degradation of the heart's ability to generate or synchronize the electrical impulses that coordinate contractions.
When the heart rate is too rapid, the condition is denoted tachycardia. Tachycardia may have its origin in either the atria or the ventricles. Tachycardias occurring in the atria of the heart, for example, include atrial fibrillation and atrial flutter. Both conditions are characterized by rapid, uncoordinated contractions of the atria.
Bradycardia occurs when the heart rhythm is too slow. This condition may be caused, for example, by delayed impulses from the SA node, denoted sick sinus syndrome, or by a blockage of the electrical impulse between the atria and ventricles. Bradycardia produces a heart rate that is too slow to maintain adequate circulation.
Implantable cardiac rhythm management systems may include pacemakers, which have been used as an effective treatment for patients with serious arrhythmias. These systems typically comprise circuitry to sense signals from the heart and a pulse generator for providing electrical pulses to the heart. Leads extending into the patient's heart are connected to electrodes that contact the myocardium for sensing the heart's electrical signals and for delivering pulses to the heart.
Pacemakers deliver low energy electrical pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency. When a pace pulse produces a contraction in the heart tissue an electrical cardiac signal associated with the heart contraction is produced, denoted the evoked response.
Pace pulses may be intermittent or continuous, depending on the needs of the patient. There exist a number of categories of pacemaker devices, with various modes for sensing and pacing the heart. Single chamber pacemakers may pace and sense one heart chamber. A typical single chamber pacemaker is connected to a lead extending either to the right atrium or the right ventricle. Dual chamber pacemakers may pace and sense two chambers of the heart. A typical dual chamber pacemaker is typically connected to two leads, one lead extending to the right atrium and one lead to the right ventricle. Bi-ventricular or bi-atrial pacemakers may be used to provide pacing pulses to both the left and right ventricles or the left and right atria, respectively. Multi-chamber pacing, including bi-ventricular and/or bi-atrial pacing may be particularly advantageous for delivering cardiac resynchronization therapy for patient's suffering from congestive heart failure (CHF).
Pacemakers can be programmed to provide pace pulses to the heart on demand or at a fixed rate. When a pacemaker paces the heart at a fixed rate, the pacemaker provides pace pulses to the heart without taking into account the heart's spontaneous action. In contrast, pacemakers may sense the spontaneous activity of the heart and provide pace pulses synchronized to the spontaneous activity.
Rate adaptive pacemakers provide pacing at rates responsive to the patient's metabolic activity. Changes in metabolic activity may reflect exercise or non-exercise related changes, such as stress or excitement. The level of metabolic activity may be determined by sensing motion, respiratory rate, QT interval, venous oxygen saturation, temperature, or other patient conditions, for example. The pacemaker automatically adjusts the pacing rate to accommodate the sensed changes in the patient's condition.
Cardiac rhythm management systems may also include cardioverter/defibrillators to provide treatment for patients with serious cardiac tachyarrhythmias. The CRM system may sense cardiac activity and recognize an aberrant fast rhythm. Upon recognition of a tachyarrhythmia, the CRM system may automatically deliver one or a series of high energy shocks to the heart, interrupting the tachyarrhythmia or fibrillation and allowing the heart to resume a normal rhythm.
In CRM systems that include sensing channels for sensing one or more heart chambers, the ventricular and/or atrial sensing channels may be blanked or rendered refractory following certain events, such as following delivery of a pacing pulse. During the blanking or refractory periods, cardiac events are not sensed by the sensing channel, or sensed events are ignored or used differently than cardiac events sensed during other periods. Sensing channels are typically blanked after pacing to prevent reentry of an output pacing pulse into the system. Sensing channels may be rendered refractory to prevent misinterpretation of input data due to sensing after potentials or crosstalk between sensing channels.
The present invention involves enhanced methods and systems for managing refractory periods for single and multi-chamber pacing and provides various advantages over the prior art.